1. Field of the Invention
The present invention relates to a magnetic detection apparatus including a giant magneto-resistance element (referred below as a GMR element), which is an electromagnetic conversion element.
2. Description of the Related Art
A GMR element is a so-called artificial lattice film, which is a laminated body in which a magnetic layer and a non-magnetic layer with a thickness of several angstroms to several tens of angstroms are alternately laminated; (Fe/Cr)n, (permalloy/Cu/Co/Cu)n, and (Co/Cu)n are known (n represents the number of layers).
Recently, an in-vehicle rotation sensor or the like has adopted a magnetic detection system that detects changes in a magnetic field acting on magneto-resistance elements by forming an electrode at each terminal of magneto-resistance segments including GMR elements as described above to configure a bridge circuit, connecting a constant voltage and constant current power source between two opposed electrodes of the bridge circuit, and converting changes in the resistance values of the magneto-resistance segments to changes in voltage.
JP-A-2004-69546 discloses such a conventional magnetic detection apparatus, which will be described below with reference to FIGS. 1A through 1C. FIG. 1A is a perspective view, FIG. 1B is a plan view, and FIG. 1C is an enlarged view of magneto-resistance segments.
In FIG. 1A through 1C, reference numeral 1 indicates a magnetic moving body such as, for example, a gear having projections around a disc and a shape for changing a magnetic field; 2 indicates a processing circuit unit having a board surface on which a circuit is printed; 2a and 2b indicate magneto-resistance segments; 3 indicates a magnet; and reference numeral 4 indicates the rotary shaft of the magnetic moving body 1. The magnetic moving body 1 rotates synchronously when the rotary shaft 4 rotates. Each of magneto-resistance segments 2a and 2b is indicated as a black block in FIGS. 1A and 1B while an enlarged view of the shape (pattern) of the magneto-resistance segment is indicated in FIG. 1C.
FIG. 2 shows the configuration of a processing circuit unit 2 of the conventional magnetic detection apparatus described above. In FIG. 2, the processing circuit unit 2 includes a Wheatstone bridge circuit 11 having the two magneto-resistance segments (GMR elements) 2a and 2b, a differential amplification circuit 12, a comparator circuit 13, and an output circuit 14. Reference numeral 151 indicates a transistor, reference numeral 152 indicates an output terminal, VCC indicates a constant voltage, and Vref indicates a reference voltage.
In FIG. 2, the constant voltage VCC is applied to a bridge circuit including the magneto-resistance segments 2a and 2b and fixed resistors 2c and 2d to convert changes in the resistance values of the magneto-resistance segments 2a and 2b caused by changes in a magnetic field to changes in voltage. A signal indicating voltage changes is amplified by a differential amplification circuit 12 and input to a comparator circuit 13. The signal is compared with the reference voltage Vref by the comparator circuit 13, converted to the final output 0 or 1 (VCC) by a transistor 151 of an output circuit 14, and then output from an output terminal 152.
Next, the operation of the conventional magnetic detection apparatus will be described with reference to FIGS. 3A through 3D which are a timing chart showing the operation of the conventional magnetic detection apparatus. FIG. 3A shows the disposition of a magnetic moving body 1, FIG. 3B shows the resistance values of the magneto-resistance segments 2a and 2b, FIG. 3C shows the output from the differential amplification circuit 12, and FIG. 3D shows the final output.
When the magnetic moving body 1 shown in FIG. 1A rotates about the rotary shaft 4, the magnetic fields applied to the magneto-resistance segments 2a and 2b is changed to cause resistance changes according to the magnetic fields applied to the magneto-resistance segments 2a and 2b corresponding to the shape of the magnetic moving body 1 as shown in FIGS. 3A and 3B.
In addition, as shown in FIG. 3C, an output from the differential amplification circuit 12 is obtained due to changes in the above resistance values. Then, as shown in FIG. 3D, the comparator circuit 13 shapes the waveform of the output from the differential amplification circuit 12 and the final output signal 1 or 0 corresponding to the shape of the magnetic moving body 1 is output.
However, there is a need for improvement of detection accuracy in a magnetic detection apparatus as described above, but the accuracy for detecting the positions of projections and depressions of the magnetic moving body 1 is limited by variations on manufacturing in the relative positions of the magneto-resistance segments 2a and 2b and a magnet 3.
FIGS. 4A through 4D schematically show a case in which the magneto-resistance segments 2a and 2b are accurately disposed relative to the magnet 3 and a case in which the magneto-resistance segments 2a and 2b are displaced in the rotation direction relative to the magnet 3. FIGS. 4A and 4B shows an example of the magnetic detection apparatus accurately disposed. FIG. 4A indicates a case in which the magnetic moving body 1 is close to the magneto-resistance segments 2a and 2b and FIG. 4B indicates a case in which the magnetic moving body 1 is away from the magneto-resistance segments 2a and 2b. In both cases, the directions the vertical direction component and the horizontal direction component) of the magnetic fields applied to the magneto-resistance segments 2a and 2b are the same and the magnitudes of the magnetic fields are also the same.
However, as shown in FIGS. 4C and 4D, when the magneto-resistance segments 2a and 2b are displaced in the rotation direction relative to the magnet 3, the in-plane magnetic field applied to the magneto-resistance segments 2a and 2b becomes uneven and the direction and the magnitude of the magnetic field become different.
As for the magnitude of the magnetic field, the magneto-resistance segments 2a and 2b have different distances from the center of the AB plane of the magnet 3. The in-plane magnetic field at the center of the AB plane of the magnet 3 is zero and the in-plane magnetic field becomes larger as the distance from the center of the AB plane increases. Accordingly, in FIGS. 4C and 4D, the magnitude of the in-plane magnetic field of the segment 2a is larger than the magnitude of the in-plane magnetic field of the segment 2b. 
As for the direction of the magnetic field, the horizontal direction component is larger in the magnetic field applied to the magneto-resistance segment 2a while the vertical direction component is larger in the magnetic field applied to the magneto-resistance segment 2b. Accordingly, the magnetic fields applied to the magneto-resistance segments 2a and 2b become uneven between the horizontal direction and the vertical direction.
Here, the characteristics of the applied magnetic field and changes in the resistance value of a GMR element (magneto-resistance element) will be described. The GMR element has significantly greater MR effects (MR change ratio) than a magneto-resistance element (MR element). In addition, the GMR element depends only on the relative angle of the orientation of magnetization of the adjacent magnetic layers, so the GMR element is an in-plane magnetic sensing element that makes the same changes in the resistance value regardless of the angle difference between the orientation of an external magnetic field and the current.
FIGS. 5A and 5B show the relationship between the strength of the magnetic field applied to a GMR element and the resistance value of the GMR element. FIG. 5A shows a case in which the magneto-resistance segments 2a and 2b are accurately disposed relative to the magnet 3 as in FIGS. 4A and 4B and FIG. 5B shows a case in which the magneto-resistance segments 2a and 2b are displaced relative to the magnet 3.
When the in-plane magnetic field is applied to the GMR element, the resistance value of the GMR element becomes different between the vertical direction (an arrow A in FIG. 4A) and the horizontal direction (an arrow B in FIG. 4A) with respect to the shape (pattern) of the GMR element. This is referred to as an anisotropy. In FIGS. 5A and 5B, the thick line indicates the horizontal magnetic field applied to the GMR pattern and the thin line indicates the vertical magnetic field applied to the GMR pattern.
The arrows in FIG. 5A indicate the ranges of the resistance value and the magnetic field of a GMR element when the in-plane magnetic field in FIGS. 4A and 4B is even. FIG. 5A shows that presence in the middle of the vertical magnetic field and the horizontal magnetic field and the magnitude of the applied magnetic field is the same in both cases. In addition, the arrows in FIG. 5B indicate the ranges of the magnetic field and the resistance value of the GMR element when the in-plane magnetic field in FIGS. 4C and 4D is uneven. The horizontal magnetic field is larger in the magneto-resistance segment 2a and the vertical magnetic field is larger in the magneto-resistance segment 2b. The magnitude of the applied magnetic field in the magneto-resistance segment 2a is larger than the magnitude of the applied magnetic field in the magneto-resistance segment 2b. Accordingly, when the in-plane magnetic field is uneven as shown in FIG. 5B, the range of changes in the resistance value is different between the magneto-resistance segment 2a and the magneto-resistance segment 2b. That is, the range of changes in the resistance value of the magneto-resistance segment 2a is lower than that of the magneto-resistance segment 2b. 
Referring to FIGS. 6A through 6D, the operation of the magnetic detection apparatus when the magneto-resistance segments 2a and 2b are displaced relative to the magnet 3 will be described. FIG. 6A shows the disposition of the magnetic moving body 1, FIG. 6B shows the resistance values of the magneto-resistance segments 2a and 2b, FIG. 6C shows the output from the differential amplification circuit 12, and FIG. 6D shows the final output.
As shown in FIGS. 6C and 6D, there are differences in changes in the resistance value of the GMR element and the output waveform of the differential amplification circuit 12 shifts in one direction. Since the electric potential value of the comparator circuit 13 does not change, the position of the output signal is displaced as compared with the case (arrow with a dotted line) in which the magneto-resistance segments 2a and 2b are not displaced relative to the magnet 3, significantly degrading the detection accuracy.